Test for 18.5.8. Set single property on a relationship
Rename GetProperty() to Property()
Test for 18.5.9. Get all relationships
Test for 18.5.10. Get incoming relationships
Test for 18.5.11. Get outgoing relationships
Test for 18.5.12. Get typed relationships
Test for 18.5.13. Get relationships on a node without relationships
Remove big Relationship test, replaced by many small tests in relationship_test.go